1. Field of the Invention
The invention relates to an electromechanical friction brake. The friction brake is intended in particular as a wheel brake for a motor vehicle. The term “electromechanical” means that it has an electromechanical actuating device, for instance with an electric motor and gear for its actuation, or in other words for pressing a friction brake lining against a brake body that is to be braked. The actuation of a brake is also called tightening. In the case of a disk brake, the brake body is a brake disk; in the case of a drum brake, the brake body is a brake drum. The invention is not limited to the disk or drum brake forms mentioned, but instead can be realized in principle in arbitrary forms of brakes. Although the invention is described below in terms of a disk brake, it is not limited to that type of brake.
2. Description of the Prior Art
From German Patent Disclosure DE 196 81 658 T1, one such friction brake is known, as a wheel brake for a motor vehicle. The known friction brake is embodied as a disk brake. As its actuating device, it selectively has an electromagnet or a linear motor, with which a friction brake lining can be pressed against a brake disk that forms a brake body to be braked. The actuating device is reinforced by a cup spring, which likewise acts on the friction brake lining in the direction of pressing against the brake body, or in other words in the direction of an actuation of the brake. Hence the actuating device needs to exert only a portion of the tightening force for pressing the friction brake lining against the brake body; the remainder of the tightening force is a spring force exerted by the cup spring. A smaller, lighter-weight and less-powerful electromechanical actuating device suffices.
The known friction brake does not have a normal cup spring but instead has a special cup spring, which has no linear characteristic spring curve; instead, its characteristic spring curve initially rises and then after a maximum drops again. After a minimum, which typically located near a zero spring force and may be positive or negative, the characteristic spring curve of the special cup spring typically rises again. If the minimum of the characteristic spring curve is negative, this means a negative spring force; that is, the direction of the spring force reverses itself. A special feature of the special cup spring is accordingly that its characteristic spring curve has a dropping portion. Conversely, in springs with a linear characteristic spring curve, the spring force, in the portion in which the characteristic spring curve drops, decreases with increasing deformation of the cup spring. In other words, the spring force of the cup spring rises with decreasing deformation, in the range in which its characteristic spring curve drops. The deformation, which can also be called deflection, means a flattening of the cup spring, which in the unloaded state is conical or curved. The deformation can continue to the point of a flat disk, or even beyond. The latter means that the cup spring is as it were inverted; that is, its curvature is reversed.
The known friction brake makes use of the special feature that the special cup spring used has a characteristic spring curve with a dropping portion. The cup spring is used in the region of its dropping characteristic spring curve; with the friction brake released, it is deformed to such an extent that its characteristic spring curve is in the range of the minimum. Upon actuation or tightening of the friction brake, the deformation of the cup spring decreases; the spring force increases, because of the characteristic spring curve that is dropping and that has moved in the negative direction. With increasing tightening of the friction brake, the spring force exerted by the cup spring on the friction brake lining thus increases, and as a result, a large proportion of the tightening force can be exerted by the cup spring. The proportion of the tightening force to be exerted by the actuating device is as a result low over the entire actuation and tightening force range.
The invention is not especially limited to a cup spring; instead, a spring element can in general be used whose characteristic spring curve drops at least over one portion. The spring element acts parallel to the actuating device; that is, the force of the actuating device and the force of the spring element are added together.